1. Field of the Invention
The present invention relates to a liquid-crystal display device having a liquid-crystal layer between one pair of substrates.
2. Description of the Related Art
In addition to the advantage of a thin and lightweight design over the cathode-ray tube (CRT) type that had formerly been the mainstream in display devices, the progress of image quality improvement technology has expanded the applications and market of liquid-crystal displays.
In recent years, the demand for better color reproducibility and higher contrast ratios in liquid-crystal displays has been increasing with the expansion of their applications as monitors for desktop-type personal computers or for printing or designing, and as liquid-crystal TVs.
For liquid-crystal display devices, since the luminance in a black state is not nil and has a finite value, the front contrast ratio of the display device is defined by the value obtained by dividing the luminance in a white state that is determined by the effective transmittance of the liquid-crystal panel, by the luminance in the black state.
Since luminance depends on a viewing angle, liquid-crystal display devices are usually discriminated in terms of front contrast ratio and viewing-angle contrast ratio. The former is the value defined by the luminance value of the panel front. More specifically, the front contrast ratio is expressed as the luminance defined by a two-degree field of view or a ten-degree field of view, that is, the luminance when measured with a luminance meter.
The latter viewing-angle contrast ratio is a characteristic determined by the luminous intensity diffusion profile of the light source, the degree of viewing-angle compensation by a polarizer, a retardation film, or the like, alignment control of the liquid-crystal molecules, or other factors. The front contrast ratio and the viewing-angle contrast ratio, however, are not completely independent characteristics, and between the two ratios, there lies the positive correlation that as the front contrast ratio increases, the viewing-angle contrast ratio also increases. The improvement of the front contrast ratio is very effective for improving the image quality of the liquid-crystal display device.
The effective transmittance in the white state is the value affected by factors such as: numerical aperture, the optical retardation and other optical constants of the liquid crystal, and if the display device is of the normally closed type, the rate of the region displayed when voltage is effectively applied.
The effectiveness of reducing the luminance in the black state and increasing the luminance in the white state in order to improve the front contrast ratio is obvious, and it is reported in Non-Patent Documents 1 and 2 that the luminance in the black state is increased by the presence of a partial depolarizer in the liquid-crystal panel.
Non-Patent Document 1: M. Yoneya et al., J. Appl. Phys., 98 (2005), p. 016106
Non-Patent Document 2: Y. Utsumi et al., EuroDisplay' 05, p. 27